This invention relates to a method for tracing an object such as an outline of a picture or a line image and following it, and in particular, relates to a method for making a mask for photo-copying.
In printing, masks are used in many ways. For example, when a catalog of goods is printed, in most cases it is made up from various photographs of the goods in different settings. Therefore, it is necessary to remove the backgrounds from these photographs before assembling them to form the catalog. Therefore, a reproduction image is obtained by preparing a film of which the necessary part is transparent and the rest opaque, laying this film on the original picture, and reproducing the combination photographically. According to circumstances, on the other hand, it may be desirable to provide such a mask in which the necessary part is opaque, and the rest transparent.
Such a mask has heretofore been made by hand. For example, in the same manner as when tracing a drawing, a transparent film sheet is laid on the original picture, and the outline of the part to be masked is drawn. The outside or the inside of the outline is then blotted out by opaque ink, to obtain the cutting mask.
This kind of hand work presents no problem, if the object to be masked has a simple outline, but if the object to be masked has a complicated, convoluted, or irregular outline, great skill and trouble, and a lot of time, are required. Hence, it is quite difficult to prepare an accurate cutting mask quickly for photographic plate making.
Another method of producing such a cutting mask is to reproduce a photofilm of the desired size, and then to directly obliterate the portion to be blanked off with opaque ink. This method again requires hand work, and suffers from cost and accuracy problems.
Another method has been used for making such a mask, in which a piece of peel-off laminate film made of a colorless transparent film and a colored transparent film which has a safe light color such as red is laid on the original picture, and then the transparent color film is cut along the outline of the object in the picture. The unnecessary part of the transparent color film is then peeled off, to obtain the cutting mask. This method skips over the process of blotting out with opaque ink, but the cutting is done by hand by a thin-bladed cutter, and therefore the outline of the image must still be followed by hand. So this method, too, is tedious, slow, unreliable and inaccurate, and expensive.
An automatic method for preparing such a mask uses an electronic color separation scanner, or a color scanner for plate making. In this case, the background of the object to be masked in the picture is set to a specific color, prior to photographing the picture. This picture is then scanned for color separation by the color scanner, and the specific color of the background is detected and exposed on a recording film to provide the cutting mask. This method is accurate and automatic, but is only applicable to an original picture of which the background has been specifically set to a given color, and cannot be used for an ordinary picture. Further, this method requires the use of a costly color scanner.
In order to remove the disadvantages and inconveniences of the conventional method, another method of tracing an outline has been developed. This method at least partially avoids these aforementioned difficulties, and is adapted to be performed at least partially by machine, so as to relieve the operator of substantial work. This method of control is adapted to be performed by machine, although it does involve some steps best done by an operator, thereby obtaining an accurate mask.
In this embodiment, as shown in FIG. 1, a sensor S which is used to perform a method of the present invention, is composed of a set of eight photoelectric detecting elements spaced around eight sectors of a circle, denoted by "1"-"8", each having substantially the same photoelectric performance. Hence, when the boundary of a picture P to be followed is curved at a right angle as shown in FIG. 2, the sensor S instead follows a curvaceous path, as shown by a broken line in FIG. 2, cutting off the corner. Further, when the area whose outline is to be followed is composed of two or more areas of different light values, then there is a danger that, when the sensor S reaches the division between two of those areas, it may take the wrong path.
In order to remove these disadvantages another method of tracing an outline has been proposed. In this method, the sensor automatically traces an outline of a boundary of different light values within a picture according to signals picked up by the detecting elements, except corner portions of the outline and intersecting portions of the boundaries of the different light values in which the outline is supplementarily traced in a manual manner after the automatic tracing is finished.
However, this method still has problems. For example, when the outline to be traced is largely curved, at least one more point of coordinates for determining the tracing direction in addition to those of the start and the destination points of a line segment to be automatically traced, of the outline should be given, and the sensor may take a wrong path when the light density difference between the picture to be cut out and and the background is small, or when the outline has a detail more than the resolving power of the sensor composed of the photoelectric elements, or when another outline exists near the outline to be traced and confuses therewith.
The method of the present invention constitutes an improvement of the method disclosed in Japanese patent laid-open publication No. 55-7846 (Japanese patent Application No. 53-131195; one of the prior applications of the current applicant). In accordance with the present invention as above discussed, data corresponding to a curve to be followed are previously memorized by coordinate reading means, such as a digitizer table, or the like, in order to enable following the outline of a curve that cannot be followed readily, or followed at all by a sensor. The output of the digitizer table or the like is recorded, the data therefrom being employed, in combination with the output of the sensor, for following the curve to produce a cutting mask.